As is generally known, in an internal combustion engine of an automotive vehicle, blow-by gases (blow-by fumes) containing unburnt gases (some air-fuel mixture), which leak down from combustion chambers into a crankcase past piston rings, are again introduced or recirculated into the combustion chambers through an intake system of the engine, together with fresh air taken in from the outside of the engine, and then combusted. The blow-by gases flowing through the crankcase contain oil mist of lubricating oil. To prevent oil mist from being carried to the intake system, an oil separator is often disposed in the cylinder head cover, so that the blow-by gases can be cleared from the crankcase after oil mist has been separated from the blow-by gases by means of the oil separator. Such oil separators have been disclosed in Japanese Patent Provisional Publication Nos. 2005-120855 (hereinafter is referred to as “JP2005-120855”) and 2009-121281 (hereinafter is referred to as “JP2009-121281”).
Two blow-by gas passages are generally connected to the cylinder head cover, such that fresh air is introduced through one of the two blow-by gas passages under a normal engine operating condition, and that blow-by gases flow through both the two blow-by gas passages under a high engine load operating condition. Hence, the cylinder head cover is equipped with two oil separators, which are used for the respective blow-by gas passages.
The oil separator as disclosed in each of JP2005-120855 and JP2009-121281 is a so-called inertial oil-mist collision type oil separator in which a partition wall having a plurality of small holes (pores or small openings or fine fluid passages) is disposed in an oil-separator chamber and also a collision plate is disposed adjacent to the partition wall in a manner so as to be opposed to the small holes of the partition wall. When blow-by gases containing oil mist pass through the small holes, the blow-by gas flow velocity increases. Thus, owing to collision of the high-velocity blow-by gas flow with the collision plate, oil mist can be recovered in the form of oil droplets adhered to the collision plate. A slit-shaped opening is also formed at the lower end of the collision plate, to enable oil droplets, adhered to the collision plate and thus separated from the blow-by gases and then gradually growing to greater particle diameters, flowing down along the wall surface of the collision plate, to flow along the bottom face of the oil-separator chamber through the slit-shaped opening toward the downstream side. In this manner, the oil can be dropped and recovered into an engine-valve operating chamber via a drain port of an oil drain pipe formed in the bottom face of the oil-separator chamber.